Page:Encyclopædia Britannica, Ninth Edition, v. 23.djvu/631

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HIS group of animals was formerly regarded as constituting along with the Polyzoa and the Brachiopoda the invertebrate class Molluscoidea. It is now known to be a degenerate branch of the Chordata, and to be more nearly related to the Vertebrata than to any group of the Invertebrata.

More than two thousand years ago Aristotle gave a short account of a Simple Ascidian under the name of Tethyum. He described the appearance and some of the more important points in the anatomy of the animal. From that time onwards to little more than a century ago, although various forms of Ascidians had been briefly described by writers on marine zoology, comparatively little advance was made upon the knowledge of Aristotle. Schlosser and Ellis, in a paper containing a description of Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the Compound Ascidians into notice; but it was not until the commencement of the 19th century, as a result of the careful anatomical investigations of Cuvier (1) upon the Simple Ascidians and of Savigny (2} upon the Compound, that the close relationship between these two groups of the Tunicata was conclusively demonstrated. Up to 1816, the date of publication of Savigny's great work (2), the few Compound Ascidians then known had been generally regarded as Alcyonaria or as Sponges; and, although many new Simple Ascidians had been described by O. F. Müller (4) and others, their internal structure had not been investigated. Lamarck (3) in 1816, chiefly as the result of the anatomical discoveries of Savigny and Cuvier, instituted the class Tunicata, which he placed between the Radiata and the Vermes in his system of classification. The Tunicata included at that time, besides the Simple and the Compound Ascidians, the pelagic forms Pyrosoma, which had been first made known by Péron in 1804, and Salpa, described by Forskal in 1775.

Chamisso in 1820 made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described by Steenstrup in 1842 as "alternation of generations"; and a few years later Kuhl and Van Hasselt's investigations upon the same animal resulted in the discovery of the alternation in the directions in which the wave of contraction passes along the heart and in which the blood circulates through the body. It has since been found that this observation holds good for all groups of the Tunicata. In 1826 H. Milne-Edwards and Audouin made a series of observations on living Compound Ascidians, and amongst other discoveries they found the free-swimming tailed larva, and traced its development into the young Ascidian. Milne-Edwards (5) also founded the group of "Social" Ascidians, now known as the Clavelinidæ, and gave a classification of the Compound Ascidians which was universally accepted for many years. From the year 1826 onwards a number of new and remarkable forms were discovered, as, for instance, some of the Bolteninæ (Macleay), Chelyosoma (Broderip and Sowerby, and afterwards Eschricht), Oikopleura (Mertens), Perophora (Lister), Pelonaia (Forbes and Goodsir), Chondrostachys and Diplosoma (Denis Macdonald), Diazona (Forbes and Goodsir), and Rhodosoma (Ehrenberg, and afterwards Lacaze-Duthiers).

In 1845 Carl Schmidt (6) first announced the presence in the test of some Ascidians of "tunicine," a substance very similar to cellulose, and in the following year Löwig and Kölliker (7) confirmed the discovery and made some additional observations upon this substance and upon the structure of the test in general. Huxley (8), in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology, and affinities of the pelagic Tunicates Pyrosoma, Salpa, Doliolum, and Appendicularia. These important forms were also investigated about the same time by Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart. The most important epoch in the history of the Tunicata is the date of the publication of Kowalevsky's celebrated memoir upon the development of a Simple Ascidian (9). The tailed larva had been previously discovered and investigated by several naturalists notably H. Milne-Edwards (5), J. P. van Beneden (10), and Krohn (11); but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not been understood. It was reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the vertebrate embryo. He showed that the relations between the nervous system, the notochord, and the alimentary canal are much the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration, and this led naturally to the view usually accepted at the present day, that the group is a degenerate side-branch from the lower end of the phylum Chordata, which includes the Tunicata (Urochorda), Amphioxus (Cephalochorda), and the Vertebrata. Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by Kupffer (12), Giard (13 and 15), and others. Important observations upon the process of gemmation and the formation of colonies in various forms of Compound Ascidians have been made by Krohn, Metschnikoff, Kowalevsky, Ganin, Giard, Della Valle, and others, and have gradually led to the establishment of the general principle, that all the more important layers of the bud are derived more or less directly from the corresponding regions in the body of the parent.

In 1872 Fol (14) added largely to the knowledge of the Appendiculariidæ, and Giard (15) to that of the Compound Ascidians. The latter author described a number of new forms and remodelled the classification of the group. The most important additions which have been made to the Compound Ascidians since Giard's work have been those described by Von Drasche (16} from the Adriatic and those discovered by the "Challenger" expedition (17). The structure and the systematic arrangement of the Simple Ascidians have been mainly discussed of recent years by Alder and Hancock (18], Heller (19), Lacaze-Duthiers (20), Traustedt (21), and Herdman (17, 22}. In 1874 Ussoff (23) investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle. In an important paper published in 1880 Julin (24) drew attention to the similarity in structure and relations between this gland and the hypophysis cerebri of the vertebrate brain, and insisted upon their homology. He suggests that they perform a renal function. The Thaliacea

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